Thinking about the hypothetical concept of time expansion mentioned in https://www.physicsforums.com/showthread.php?t=389378" made me consider something for the first time: The primary signature of spatial expansion is, of course, redshift of the light that reaches us from the distant universe (i.e. the distant past). However, an even more immediate consequence of that expansion is the dilution of matter. What I'm wondering now is whether that dilution is directly observable, i.e. whether we observe the early (transparent) universe to be denser than today's universe. At, say, z=1, we can assume the structure of the universe to be very similar to today's (right?), but its density should be almost ten times higher. At z=4, the density should be more than a hundred times higher, but I'm not sure how far galaxy formation has progressed at this point, which might complicate matters. Anyway, an x times higher density should mean an x times higher concentration of galaxies and/or an x times denser IGM. Transverse to our line of sight, I imagine one would have to take optical effects into account. To resort to the inflating-balloon analogy and assuming no horizon for the moment, the highest-redshift objects we observe in any direction would be those on the far side of the balloon, i.e. we would observe the same small region stretched into a large annulus. Radially, this does not occur, but a multitude of other factors affecting the light between emission and reception have to be taken into account. So I'm guessing a direct observation is much harder to accomplish than one might naively assume... or is it?